The present invention relates to an electrical discharge machining method and apparatus therefor that uses an electrode of a simple shape, such as a tube, a cylinder or an angular column, and that realizes three-dimensional machining by NC control.
There is conventionally known an electrical discharge machining apparatus which three-dimensionally controls an electrode of a simple shape, such as the shape of a tube, a cylinder or an angular column, by NC control means to form a desired three-dimensional shape. In this type of electrical discharge machining apparatus, it is unnecessary to fabricate a compound die electrode of a complicated three-dimensional shape, thereby significantly reducing manufacturing costs for a metal die and reducing the manufacturing time. Moreover, since the electrode shape can be defined in advance, it is easy to construct a CAM system, and automation of machining steps is expected.
However, this type of an electrical discharge machining apparatus has problems with respect to electrode wear and accuracy of machining, in comparison with electrical discharge machining utilizing a compound die electrode.
In order to solve these problems, for example, the engineering department of Yamagata University has proposed a method for dividing a desired three-dimensional shape into several layers in a Z-axis direction and carrying out machining of each layer while simultaneously correcting electrode wear.
For example, FIG. 42 is a constitutional view of an electrical discharge machining apparatus such as disclosed in "Three-dimensional Control Electrical Discharge Machining by a Tubular Electrode (third report)", Electrical Processing Society Journal, Vol. 17, No. 34, pp. 30-42 (1984), which is one of a series of research reports by Tsuchiya, Kaneko et al.
In the FIG. 42, reference numeral 1 is an electrode of a cylindrical shape or the like, 2 is a workpiece as a processed material, 3 is an X-Y table for fixing the workpiece 2, 4 is an electrode rotating mechanism for rotating the electrode 1 about a Z-axis, 5 is an X-axis driving means for driving the X-Y table 3 in an X-Y direction, 6 is a Y-axis driving means for driving the X-Y table 3 in a Y direction, 7 is a Z-axis driving means for driving the electrode rotating device 4 with the electrode 1 attached in the Z-axis direction, 8 is a working power source for supplying working pulses between poles formed by the electrode 1 and the workpiece 2, 9 is a machining state detecting means for detecting a machining state during machining, 10 is an NC control means for controlling the X-axis driving means 5, Y-axis driving means 6 and Z-axis driving means 7, 11 is an electrode wear correcting means for correcting wear of the electrode 1 during machining by a positional information. The NC control means 10 gives appropriate commands to each of the X-axis driving means 5, Y-axis driving means 6 and Z-axis driving means 7, while stably maintaining the machining state detected by the machining state detecting means 9 on the basis of a three-dimensional locus command instructed by the electrode wear correcting means 11.
Mr. Tsuchiya and Mr. Kaneko and others have implemented the NC control means 10 and the electrode wear correcting means 11 by a program using a microcomputer. Still, though this implementation does not cause any limitation to the electrode wear correcting method.
FIG. 43 is an explanatory view of the operation of the electrode wear correcting method showing a conventional process, after a three-dimensional machining shape desired to be machined is given, until NC data provided with an electrode wear correction is obtained, which has been proposed by Tsuchiya, Kaneko et al.
First, a process 12a provides data of a desired three-dimensional shape to be machined. Next, a process 13a slices the data of the three-dimensional machining shape into a plurality, e.g., in the Z-axis direction, thereby dividing it into some layers. A series of processes 14a enclosed by a dotted line in the figure is applied to each layer.
Now, it is supposed that the thickness of one layer divided herein is E. A process 15a generates a path of the electrode in an X-Y plane, namely a tool path. Then, a process 16a executes a machining program corresponding to the thickness E of this layer in the Z-axis direction at the start point of the tool path. Thereafter, a series of processes 19a and 20a are carried out to perform machining in the X-Y plane while correcting wear of the electrode.
Next, a principle of the electrode wear correcting method shown in the process 19a is described based upon experimental results of Tsuchiya, Kaneko et al.
FIG. 44 is a graph showing the relation between an electrode moving amount and an electrode consumed length in the X-Y plane in a case where electrode wear correction is not carried out. FIG. 45 is a graph showing the relation between the electrode moving amount and the electrode consumed length in case of machining while performing the electrode wear correction.
Seeing the relation between the electrode moving amount and the electrode consumed length shown in FIG. 44, it is understood that the electrode consumed length is negative at the beginning of machining, and the electrode consumed length increases gradually thereafter. With respect to the amount of change of the electrode consumed length, it is also understood that, the curve inclination changes between m1, m2, m3 depending on the shape or material of the electrode, the material of the workpiece, electrical conditions and the like, and if the change of the electrode consumed length is over a predetermined value, thereafter the electrode 1 is consumed at the same rate. Therefore, the point at which the electrode consumed length changes from negative to positive is made as a correction start point Lc. Then, a correction amount in the Z-axis feeding direction is obtained for each appropriate correction reference interval .DELTA.L, e.g., on the basis of the curve inclination m2.
The correction amount .DELTA.LE of the feed in the Z-axis direction is: EQU .DELTA.LE=mi.multidot..DELTA.L
where:
mi: curve inclination of electrode consumed length PA2 .DELTA.L: correction interval PA2 Lc: correction start point PA2 Thus, the machining program with a Z-axis feed correction is executed in the process 20a.
As shown in FIG. 45, if electrode wear correction is performed, the total electrode consumed length increases linearly. This shows that, even if there is wear of the electrode, machining in which the machining depth is uniform, namely, machining of one layer divided into a thickness E, is possible.
Here, parameters necessary for electrode wear correction shown in the process 19a should be stored beforehand as machining technique data 18a corresponding to the thickness E of one divided layer given by the process 13a and the shape or material of the electrode, material of the workpiece, electrical conditions and the like, which are given by the process 17a. Since the electrode consuming amount has a close relation with the electrode shape during machining, the correction amount of electrode wear should be decided in consideration of the case in the removing amount during machining changes according to the tool path.
Japanese Laid-Open Patent Publication No. 5-345228 describes an electrode wear correcting method on the basis of a similar concept to the above.
FIG. 46 is an explanatory view illustrating the principle of a conventional electrode wear correcting method.
As shown in the figure, a tubular electrode is fed in a slanting direction by an angle a relative to a plane, that is, processed by electrical discharge, while being rotated, so that it is possible to attain a stationary state from a position (c) to and after a position (d), in which the profile shape of the electrode 1 and machining depth do not change, after passing a transient state from a position (a) to the position (c), in which the profile shape of the electrode 1 and the machining depth change. At that time, if the machining has a condition of a large electrode consuming amount, the transient state from the position (a) to the position (c) can be nearly ignored. Accordingly, it is possible to carry out removal machining for a layer shape whose machining depth is constant by feeding the machining electrode in the slanting direction by a suitable feeding angle .alpha..
At that time, after a short period of early transferring time wherein the electrode 1 touches the workpiece 2 and the profile changes, the profile of the electrode 1 is stable (unchanged) in the rest time of machining (between the position (d) to the position (c)), and the electrode length is consumed while only the electrode length decreases. In the illustrated example, the shape of the leading end of the electrode becomes conical at the end of the transferring step, and an inclination angle .beta. is dependent on the layer thickness E (cutting depth or groove depth) and radius R of the tubular electrode.
FIG. 47 is an explanatory view of an electrode wear correcting process illustrating the principle of a conventional electrode wear correcting method.
First, a desired three-dimensional machining shape to be machined is inputted in a process 12b. Next, the three-dimensional machining shape is divided into several layers in a process 13b. Then, a series of processes 14b enclosed by a dotted line in the figure are applied to each layer. Now, it is supposed that a thickness of one divided layer is E. A process 15b generates a path of the electrode 1 in the X-Y plane, namely, a tool path. A series of processes 19b and 20b form a machining program with an electrode wear correction performed prior to the machining for the tool path. According to the principle shown in the FIG. 46, an inclining feed angle .alpha. of the electrode for removing one layer of the thickness E can be obtained by the following expression, as shown in the process 19b, from the layer thickness E, a radius R of the machining electrode, a cross sectional area S of the machining electrode and a volume consuming rate U, in consideration of the machining amount and the electrode consuming amount in the stationary state. EQU tan(.alpha.)=R.multidot.E.multidot.U/S (1)
wherein the expression (1) corresponds to a case in which the electrode 1 is a tubular shape as shown in FIG. 46. In case the electrode 1 is a hollow tubular shape, the inclining feed angle .alpha.(alpha) of the electrode is shown by the following expression, supposing that the machining electrode has an outside radius R1, an inside radius R2 and a cross sectional area S. EQU tan(.alpha.)=(R1+R2).multidot.E.multidot.U/S=E.multidot.U/.pi./(R1-R2)(2)
Accordingly, it is necessary to derive expressions for correcting the electrode wear corresponding to the shape of the electrode 1. Here, the parameters can be selected from machining technique data 18b that is prepared beforehand on the basis of a layer thickness given in the process 13b and the shape or material of the electrode, material of the workpiece, electric conditions and the like given in the process 17b.
As mentioned above, the technique disclosed in the publication employs a simulator that calculates a value for correcting longitudinal wear and calculates a feed angle a of the electrode relative to the plane of a layer that is processed by electrical discharge, by moving through a thickness E of a removed layer, a radius R of an electrode and a consumed volume amount U, thereby performing machining by an inclining movement. Particularly, it is shown that the technique can correct longitudinal wear of the electrode by feeding it in the slanting direction, so that it is possible to use an electrode consuming area where the machining speed is increased, thereby improving a work efficiency.
As mentioned above, in the electrical discharge machining apparatus for machining a desired three-dimensional shape by three-dimensionally controlling the electrode 1 of a simple shape like a cylinder or angular rod by use of the NC control means 10, it is important how accurately and simply the electrode wear correction is performed.
However, in the electrode wear correcting method reported in the former conventional "Three-dimensional Control Electrical Discharge Process by a Tubular Electrode (third report)", it is necessary to experimentally measure the correction start point, correction reference interval and curve inclination according to a variety of layer thicknesses, shape or material of an electrode, material of the workpiece, electrical conditions and the like. Moreover, there is a problem that it is very troublesome to store these large quantity of data as machining technique data and so on.
In the electrode wear correcting method described in the latter Laid-Open Patent Publication No. 5-345228, it is theoretically possible to decide an inclining feed angle of the electrode analytically prior to machining if the layer thickness, radius of the machining electrode, cross-sectional area of the machining electrode, and volume consuming rate are given as machining technique data. However, since the electrode consuming rate for practical machining conditions varies depending on the machining liquid temperature or machining chip amount in the machining gap, it cannot always remove accurately a required layer thickness. Therefore, there is a problem that it is troublesome to correct the machining technique data and make a machining program for additional machinings. Namely, it is necessary to input all correcting amount of Z-axis as a Z-axis feed command of an NC program before starting machining. Accordingly, the NC program becomes very complicated and needs a large capacity. At the same time, it is difficult to change the correcting amount in Z-axis even if the machining state changes during machining.
Therefore, the present invention has been made to solve these problems, and it is a first object thereof to reduce the amount of machining technique data which must be inputted manually and which requires correction for electrode wear and to realize electrode wear correction with an easier method.
On the other hand, the electrical discharge apparatus using an electrode of simple shape can perform machining by the electrode of simple shape without fabricating an electrode of complicated shape. However, the machining speed decreases due to an area effect. That is, generally, an electrical discharge machining has an upper limit of an electric current value that can be applied thereto, which varies in accordance with an electrode area. If electric current over such value is supplied, an abnormal arc is generated and machining becomes impossible. If the electrode area is small, it is experimentally verified that this limit value decreases. Usually, the larger the electrode area, the higher the current density can be, thereby increasing machining efficiency. This phenomenon related to the electrical discharge machining is called an area effect. In conventional machining using the electrode of simple shape, the consuming condition can be used, so that the limit value of applied electric current rises. Still, there is a problem in that, under normal electrical discharge machining conditions, the machining efficiency is rather low in comparison with the compound die.
Moreover, a conventional and common electrical discharge machining apparatus for die sinking uses oil as a working fluid, while a wire electrical discharge machining apparatus uses water.
The following are reasons why electrical discharge machining using water is unsuitable for a die sinking electrical discharge machining apparatus and why water has not been used as in the wire electrical discharge machining apparatus.
(1) Low machining accuracy
Since a common electrical discharge machining takes transfer machining by a compound die as a premise, low-wear machining is indispensable. In electrical discharge machining by water, it is known that low electrode wear cannot be obtained by a reversed polarity (electrode positive) as in machining by oil, and that low electrode wear can be obtained at an area of a large pulse width of a positive polarity (electrode negative). (See Kimoto and Tamiya, Electrical Processing Society Journal, Vol. 3, No. 5 (1969), pp. 23-29, "Electrical Discharge Machining of Low Electrode Wear in Water (I)"). It has been clarified that a condition to get a low wear is very narrow and critical (See Masuzawa, Electrical Processing Society Journal, Vol. 14, No. 27 (1980), pp. 50-57, "Study of Electrical Discharge Machining Using Water as Machining Liquid (first report)"). Accordingly, in case water is used as a working fluid, it is generally difficult to maintain a low consuming state of the electrode. In this regard, the wire electrical discharge machining apparatus feeds the electrode successively, so that electrode wear can be ignored in machining. However, it is hard to obtain a high machining accuracy in case of die sinking electrical discharge machining by the use of the compound die.
(2) Slow machining speed
As long as the low electrode wear condition is used, if water is utilized as working fluid, the machining speed is lowered approximately one-half to one-third in comparison with the oil.
The present invention was made to solve the abovementioned problems, and it is a second object to obtain an electrical discharge machining apparatus that can obtain a higher machining speed and machining accuracy than has been the case conventionally in three-dimensional machining by the use of an electrode of simple shape.
FIGS. 48a to 48f are process explanatory views showing examples of a machining path illustrated in Japanese Laid-Open Patent Publication No. 5-345228. FIGS. 49a and 49b are explanatory views consisting of a plan view (FIG. 49a) and a front view (FIG. 49b) in the case of performing corner portion finishing by side surface machining.
In this type of machining, flashing and unremoved portions are produced along the edge of a layer that has been machined by electrical discharge. Therefore, it is necessary to vary the pattern of the machining path each time the layer changes from one to another in order to remove material left along the edge. Namely, as illustrated in FIGS. 48a to 48g, machining of a desired depth and shape is made possible by repeatedly performing a removing machining operation on the layers many times while switching various machining path patterns.
In the conventional electrical discharge machining apparatus using an electrode of simple shape, it is necessary to vary the machining path pattern each time the layer changes from one to another in order to remove material left along the edge of the machined layer. Therefore, for example, as illustrated in FIGS. 48a to 48g, it is necessary to generate machining paths (machining programs) for repeatedly performing the removing machining of the layers while switching various machining path patterns. Thus, there are problems that the machining programming is made complicated and that the required data capacity is made very great.
Furthermore, even if the machining path is repeatedly machined, the shape which can be machined is limited to a cavity shape of 2.5 dimensions whose side surface is vertical. It is hard to machine a cavity that has a three-dimensional shape as its side surface, having a tapered surface and a curved surface.
In addition, a smallest corner equal to the radius R of a cylindrical electrode or tubular electrode is formed at an inside corner portion of a cavity shape machined by the cylindrical electrode or tubular electrode. It is difficult to finish this corner portion. That is, in the conventional machining by an electrode of simple shape, the machining is performed while keeping a bottom surface shape of the electrode in a stationary state by performing a consuming machining at the bottom surface portion of the electrode. However, after rough machining, as illustrated in FIGS. 48a to 48g, if the consuming condition is used in carrying out a conventional automated enlarging or drawing machining (finishing machining by a side surface portion of the electrode), an electrode radius is reduced due to wear in the cylindrical or tubular electrode. Moreover, a corner portion is worn in the pattern of a square electrode. Accordingly, there is a problem that the shape accuracy at the corner portion is extremely deteriorated.
Therefore, it is necessary to switch the electrical machining condition to a low consuming condition in order to perform good finishing of the corner portion by side surface machining, as shown in FIGS. 49a and 49b. However, it is common that the pulse width of the electric current pulse is increased for the low consuming condition, so that the surface roughness at the corner portion is made worse. Otherwise, the machining speed must be drastically decreased in order to maintain the surface roughness. Moreover, even in case of using the low consuming condition, there arises a problem in the case of a square electrode in that the shape accuracy worsens due to wearing of the electrode corner.
Therefore, the present invention has been made to solve the above conventional problems, and it is a third object to provide an electrical discharge machining method and apparatus therefor that makes programming easy, improves the machining shape accuracy at an edge portion, can easily perform side surface machining of a three-dimensional shape, and can improve a machining accuracy at corner portions.